The present exemplary embodiment relates generally to determining a flatness characteristic for a reflective facet of a polygon assembly. It finds particular application in a dynamic test setup suitable for testing a polygon assembly or a motorized polygon assembly (MPA) at a normal operating speed that the assembly under test is expected to experience during operation of system equipment with a raster output scanning (ROS) subsystem that includes the assembly under test. The dynamic test can determine the curvature of any individual reflective facet for the assembly under test. The deviation of facet-to-facet flatness for the assembly under test can also be determined. The facet curvature and facet-to-facet deviation can be compared to predetermined thresholds to determine if the assembly under test is suitable (or not suitable) for use in a ROS subsystem. However, it is to be appreciated that the exemplary embodiments described herein are also amenable to testing polygon assemblies or motorized polygon assemblies for other purposes. For example, the dynamic test may be used to test capabilities of experimental designs, to prove principles of new designs, to accept tooling for manufacturing runs, to confirm assemblies are suitable for sale by a manufacture or supplier, or to accept procurement of assemblies from a manufacture or supplier.
Facet-to-facet flatness of a polygon assembly is an important consideration to maintaining image quality. Deformation of even a single facet can result in unwanted image artifacts, such as banding. Historically, evaluation of polygon surface flatness has generally been confined to interferometric measures on static assemblies. While this provides useful information, it does not necessarily provide an accurate account of the flatness during operation of the a ROS subsystem in system equipment, such as a laser printer. For example, centrifical forces exerted on the polygon assembly during rotation can result in facet deformation. Therefore, techniques for dynamically determining flatness characteristics of facets are desirable.
A previous solution for testing a polygon assembly using a dynamic test setup is described in Kessler, et al., “Dynamic Optical Tests High Speed Polygon,” Applied Optics, Vol. 20, No. 6, Mar. 15, 1981, pp. 1015-1019, the contents of which are fully incorporated herein by reference. The Kessler dynamic test setup uses an astigmatism generator with dual parallel plates inserted into the optical path at equal, but opposite, angles and a Ronchi with both vertical and 45 degree lines. The astigmatism generator angle is adjusted while moving a lens (L1) back and forth until the modulation patterns from both the vertical and 45 degree patterns peak at the same time. The polygon surface error is then calculated based on the rotation angle of the astigmatism generator. The Kessler approach was attempted and repeatability was found to be on the order of approximately 30 micrometers (1 sigma). This degree of repeatability is not acceptable. Therefore, improved techniques for dynamically determining flatness characteristics of facets are desirable.